Cycloolefin copolymer resins have been utilized in optical applications due to their superior transparency. In addition, since cycloolefin polymers have favorable heat resistance and water vapor barrier properties, they have attracted attention in fields of packaging materials and sheets for general industry. In particular, taking advantage of amorphous polymer features, a process such as vacuum molding or air-pressure molding in which a sheet or a preform is heated to the softening treatment temperature around the glass transition point and the molding is carried out.
However, since cycloolefin copolymer resins exhibit an abrupt change of softening behavior before and after glass transition during heating, for example, local breaking of film sheets in use at around the glass transition point and in softening molding, and rupture of preforms in injection stretch blow molding can occur.
In order to control the flexibility of a cycloolefin resin, several combinations of cycloolefin resins having different glass transition points have been attempted. For example, Japanese Unexamined Patent Application, First Publication No. 9-278974, the entire disclosure of which is incorporated herein by reference, discloses that a thermally shrinkable cycloolefin polymer tube constituted with a composition having a main glass transition temperature falling within the range of 55 to 90° C. that includes a cycloolefin random copolymer mixed with an olefin resin having a storage modulus of no less than 5×109 dyn/cm2 as measured at a frequency of 10 Hz and a temperature of 30° C. is superior in stiffness.
In addition, Japanese Unexamined Patent Application, First Publication No. 11-80492 discloses that a stretch blow molded bottle composed of a cycloolefin copolymer composition including a cycloolefin copolymer having a glass transition point of 140 to 105° C. blended with a cycloolefin copolymer having a glass transition point of 100 to 60° C. is superior in balance of impact resistance and heat resistance, and also superior in transparency after subjecting to hot filling.
However, these prior art references postulate molding and practical use in a comparatively low temperature range, and use in an environment having a temperature exceeding 140° C. is impossible. For example, mold release films for use in a curing die of thermosetting resins are included.